The investigation of superconductor devices has been extremely active in recent years because of the potential benefits that can be achieved if their performance and production methods can be made practical for reliable and economical applications in the electronic fields. A tunnel junction and microbridge are the two basic active Superconductor devices which exhibit the ac and dc Josephson effects are required for many applications. Although tunnel junctions fabricated from the conventional, low temperature superconductors have proven to be quite useful, severe surface and interface problems have prevented the successful fabrication of high quality, high temperature superconductor tunnel junctions to date. The extremely short coherence lengths in YBa.sub.2 Cu.sub.3 O.sub.7-x, .about.2-15 .ANG., place stringent requirements on the quality of the tunnel barrier material and its interfaces in superconductor-insulator-superconductor (SIS) device structures. These requirements include that the barrier material be extremely thin (.about.10-20 .ANG.), pin hole free, chemically compatible with the superconductor, and that the superconducting energy gap be fully developed within a coherence length of the interfaces.
The fabrication of high Tc superconductor/normal metal/superconductor (SNS) microbridges are believed to be less difficult to fabricate than tunnel junctions, due to fewer problems with material interaction and because the normal metal weak link layers can be thicker and hence more easily controllable than the typical tunnel barriers. The stringent requirements for SIS structures are thus somewhat relaxed for SNS structures, though the requirement of high quality interfaces remains. Unfortunately, high quality interfaces are difficult to attain since most metals are reactive with orthorhombic YBaCuO, (o-YBaCuO) and use of a nonreactive Ag or Au makes growth of a high quality of o-YBaCuO counter-electrode difficult due to the mismatch between the o-YBaCuO and Ag or Au crystal lattices.
All high Tc Josephson weak links are potentially useful for high frequency sources and detectors, for high speed, low power digital logic, and for sensitive magnetic field detectors. For optimal performance in these applications, certain device characteristics are desirable, including large (&lt;1 mV) values of the critical current-normal state resistance product (I.sub.c R.sub.n) accompanied by strong ac and dc Josephson response, device resistances on the order of 50 .OMEGA., and high critical current densities (J.sub.c) for high speed operation. A great deal of effort has been focused on obtaining useful all high Tc Josephson devices with published reports on a wide variety of device structures. The various junction geometries that have been studied include grain boundary weak links as reported in Mannhart et al. "Critical Currents in 001! Grains and Across Their Tilt Boundaries, in YBa.sub.2 Cu.sub.3 O.sub.7 Film", Phys. Rev. Lett. 61, pg. 2476 (1988) and Russek et al. "Scaling Behaviour of YBa.sub.2 Cu.sub.3 O.sub.7-x Thin Film Weak Links", Appl. Phys. Lett. 57, pg. 1155 (1990), all YBa.sub.2 Cu.sub.3 O.sub.7-x step edge microbridges, see Simon et al., "Engineered HTS Microbridges", IEEE Trans. Magnetics 27, pg. 3209 (1991), Au or Ag-coupled microbridges, and epitaxial sandwich or edge-geometry structures using barriers such as PrYBa.sub.2 Cu.sub.3 O.sub.7-x, Bn-doped SrTiO.sub.3, Bi.sub.2 Sr.sub.2 CuO.sub.y, or CF.sub.4 treated YBa.sub.2 Cu.sub.3 O.sub.7-x, see Gao et al, "Preparation and Properties of All High T.sub.c SNS-Type Edge DC Squids", IEEE Trans Magnetics 27, pg 3062 (1991), Chin et al., "Novel All-High T.sub.c Epitaxial Josephson Junction", Appl. Phys. Lett. 58, pg. 753 (1991), Mizuno et al., "Fabrication of Thin-Film-Type Josephson Junctions Using a Bi--Cr--Ca--Cu--O/Bi--Sr--Cu--O/Bi--Sr--Ca--Cu--O Structure", Appl. Phys. Lett. 56, pg 1469 (1990), and Koren et al., "Properties of all YBa.sub.2 Cu.sub.3 O.sub.7-x Josephson Edge Junctions Prepared by in situ Laser Ablation Deposition", Appl. Phys. Lett. 58, pg. 634 (1991). Although progress has been made with these devices, their electrical characteristics are often less than ideal with many devices suffering from one or more of the following problems: 1) current-voltage (I-V) characteristics inconsistent with the resistively-shunted junction (RSJ) model; 2) weak magnetic field and microwave response; and 3) low I.sub.c R.sub.n products (&lt;500 .mu.V).
U.S. Pat. No. 4,891,355 discloses a method of producing a superconducting circuit, wherein a laser beam is used to form nonsuperconducting barrier regions between superconducting portions of the film. These barrier regions appear to electrically separate various superconducting parts of the circuit and the dimension of the nonsuperconducting segments will be limited to the dimensions of the laser wavelengths, due to diffraction effects. In essence, a film of a ceramic superconductive material is deposited on a substrate in a superconducting state and then exposed to the laser beam as part of a high temperature heat treatment to turn it into a nonsuperconducting state.
An article "Cubic Phase in the Y--Ba--Cu--O System" by Agostinelli et al., Physics Review B 43, pg. 11396 (1991), discloses a cubic phase of a rare earth-alkaline earth-cuprate film that was deposited as a thin film on a single crystal magnesium oxide substrate by excimer laser ablation.
U.S. Pat. No. 5,034,374 teaches a method of producing a high temperature superconductor element formed from rare earth-alkaline earth-cuprate superconductors with an insulating layer formed between the lower and upper ceramic high temperature superconductor films by an interdiffusion of ceramic superconductors to thereby form a tunnel junction exhibiting Josephson effects.
U.S. Pat. No. 5,047,390 discloses another configuration of a tunnel junction Josephson device.
Finally, the Japanese laid-open patent application No. 63-252316 apparently refers to the deposition of a barrier layer between a substrate and a superconducting material.
The prior art is still seeking to provide an optimum superconducting device exhibiting Josephson effects that can be efficiently manufactured with high yield.